Current Line Drive (CLD) – the advantages
The long cables used in permanently installed monitoring systems must stand up to considerable electrical and mechanical interference. With traditional sensors the signals barely get through the network, being drowned out by the noise and interference.
The solution: use either expensive high-quality cable carefully laid away from interference sources or a ‘line drive’ system, which consists of a tiny electronic amplifier built into each sensor which boosts the vibration signal.
The latter offers several advantages:
- Low sensitivity to mechanical and electrical interference (cable noise, electromagnetic sources, ground looping)
- Very long low-cost cables possible with very little signal loss
- Cable positioning during installation is not as critical
- Power supply current carried along the very same coaxial cable carrying the vibration signal (power comes from a source built into the receiver instrument).
There are two types of line drive systems, providing either voltage output or current output. PRÜFTECHNIK systems use the latter, since it is a superior system with more sophisticated electronics and the following significant advantages:
- Much lower high frequency loss in very long cables even over 1000 meters.
- Much lower susceptibility to induced noise and ground-loop noise, also obviating in most cases the need for insulated sensors.
- High frequency loss in long cables
The PRÜFTECHNIK current line driver system has much lower high frequency loss in long cables than voltage output systems. This is because the instrument has a significantly lower input impedance, and since the maximum frequency is inversely proportional to input impedance, the maximum frequency is greatly extended.
Ground looping and noise
The PRÜFTECHNIK current line driver system has much lower susceptibility to induced noise and ground-loop noise than voltage output systems. This is because the very low instrument input impedance minimizes influence from electromagnetic fields. The instrument virtually ‘shorts’ the cable conductors.
This has the added advantage that, with the exception of frequency rectifier-governed motors and high-voltage motors, costly insulated sensors can be avoided, eliminating the problem of insulator capacitance.
